The field of the invention is industrial control systems such as process controllers, numerical controls and programmable controllers, and particularly, communication systems for connecting such controllers together in a network.
Historically, industrial control communications networks have been organized into star, multidrop, or ring configurations. In a star configuration, one controller, or "node", forms the center and acts as the network "master". Separate lines extend from this master node to all of the other "slave" nodes. In a multidrop network such as that disclosed in U.S. Pat. No. 4,319,338, a communication line, or data trunk, connects to a plurality of nodes by drop lines. In multidrop networks a single node may act as the network master, mastership may be transferred between nodes by passing a "token", or mastership may be transferred between nodes after a polling process determines which node requires control of the network. Also, in some multidrop networks contention schemes are employed to avoid the need for identifying a master node. In such systems each node "contends" for access to the data trunk, and when access is obtained, a message may be sent directly to any other node in the network. Special hardware is required to detect "collision" of messages on the data trunk.
With a ring configuration each node is linked to two other nodes in a loop arrangement. Messages are relayed around the loop from one node to the next. Messages are removed from the ring by the destination node, or if the message travels full circle, it is removed by the originating node. Because all communications is lost if any single node fails in a ring network, it is common practice to provide a second, or redundant, ring which is switched into operation if the primary ring fails. In a typical installation the primary ring and secondary ring are located adjacent to each other, and if an accident occurs which damages the data link for one ring, it is likely that the other data link will be damaged as well. When both rings are damaged, the entire communication system is closed down.